This invention provide a method of preventing surface cracking on Ni containing, continuously cast steel products for service at low temperatures.
The continuous casting process has been remarkably developed in the steel making processes, since it omits the ingot-slabbing steps, to save the energy and the man power, or to increase the yield. The continuous casting process has qualitatively and quantitatively widened its available fields, and has been applied to Ni steel (5.5 to 10% Ni) such as 9%Ni steel and others for low temperature service.
However, the continuous casting of Ni steel has one serious problem. This is that the continuously cast steel products containing 5.5 to 10% Ni have many defects such as surface cracking on the steel product in comparison with low alloy steel, and therefore it necessitates complicated surface conditioning treatment such as cold scarfing or low degree slabbing as a pre-process to a hot rolling operation in a subsequent process. These treatments act as obstacles so that the above mentioned merits could not be satisfactorily displayed.
In regard to causes of the surface cracking, it is in general known that, under a condition that the .gamma. (austenite) grain boundary is embrittled by second phase (sulfides or nitrides) precipitating at the .gamma. grain boundary, when tensile stress exceeding a certain limit is loaded about the steel surface, nuclei of voids or pores are generated as encircling the second phase, and those voids link up with one another and finally cause cracks. Since in the continuous casting process there is generated stress in the continuously cast steel between the rolls in the cooling zones, or thermal stress by repetition of cooling and heat recuperation, surface cracking is more easily caused than in a conventional ingot casting process.
To decrease the surface cracking on the continuously cast products such as billets, slabs, blooms and so on (briefly called as "slab" representatively hereinafter), the prior art has adopted methods of controlling requirements such as the casting temperatures or speeds, or controlling demands such as the amount of cooling water in the secondary cooling zone, or using electromagnetic stirring. Even if limitation is provided as to the casting condition or the cooling condition with respect to Ni steel, the occurrence of the surface cracking could not be prevented.
In view of these circumstances, the present invention has been proposed through many investigations and studies.
An object of the invention is to provide a method of manufacturing Ni containing steel slabs for low temperature service by the continuous casting process, without providing any limitations as to the casting condition or the cooling condition, whereby to reduce or eliminate the surface cracking on the continuously cast steel slab so that the surface conditioning treatment prior to the final rolling is no longer required.
For accomplishing the object, the investigations have been carried out about the cause of the surface cracking and the countermeasure thereto for a long term, in which, by specifying the chemical composition of molten steel to be continuously cast, it was succeeded to obtain steel cast slabs with no surface cracking and not requiring any treatment for removing these surface cracks.
That is, the invention is characterized in that, for continuously casting 5.5 to 10% Ni containing steel, the chemical composition of the molten steel is adjusted to provide S less than 0.0020%, N less than 0.0045% and Ca 0.0020 to 0.0070%, and further characterized in that Ti content is adjusted 0.005 to 0.015%, and such molten steel is continuously cast.
Hereafter, the present invention will be explained with reference to the attached drawings.